(1) Field of the Invention
The present invention relates to a double flooring structure which may be used to constitute the floor of a computer room or the like in a building, that is, a free access floor. More particularly, the present invention pertains to a free access floor which is defined by a multitiplicity of floor panels laid above the floor surface of a building so that a space for accommodating cables, ventilating ducts and the like is ensured between the free access floor constituted by floor panels and the floor surface. The present invention is also concerned with a method of constructing said free access floor.
(2) Description of the Prior Art
In general, a free access floor which may be used for a computer room or the like is laid at a level 500 mm or more from the floor surface of the building. A large number of cables are connected to a computer and it is also necessary to install ducts or the like, for example, ventilating ducts of an air conditioner, and therefore a sufficiently large space to accommodate such cables and ducts must be ensured under the floor panels that constitute the free access floor. In order to ensure a sufficiently large underfloor space, floor panels used to constitute a free access floor are laid in such a manner that support legs, which are members separate from the floor panels, are stood on a floor surface made, for example, of concrete, and the floor panels are supported on the upper ends of the support legs.
FIG. 1 shows a conventional free access floor of the type in which floor panels are supported by means of support legs which are members separate from the floor panels. In the case of the illustrated free access floor, bases 2 are firmly secured by means, for example, of an adhesive at predetermined positions, respectively, on a floor surface 1 of a building, and the lower ends of support legs 3 are secured to the respective bases 2 by means, for example, of welding. A height adjusting plate 4 is brought into thread engagement with the upper end portion of each of the support legs 3 standing on the respective bases 2. A plurality of floor panels 5 are supported on the height adjusting plates 4 in such a manner that the floor panels 5 are disposed adjacent to each other. The floor surface 6 of the free access floor is defined by the upper surfaces of the floor panels 5.
Since in the above-described free access floor the support legs 3 are firmly secured to the floor surface 1 through the bases 2, when lateral force is applied to the free access floor due to an earthquake or the like, the bending moment that acts on the free access floor reaches a maximum at the lower end of each support leg 3, as shown in FIG. 2. In other words, the earthquake resistance of the free access floor depends on the bond strength between the floor surface 1 and the bases 2. Therefore, it is necessary in order to obtain satisfactory earthquake resistance to interpose a reinforcing rod 7 between the floor surface 1 and the intermediate portion of each support leg 3 to thereby prevent collapse of the support legs 3, as shown in FIG. 1.
It should be noted that, although each pair of adjacent height adjusting plates 4 are connected together through a stringer 8 in the prior art shown in FIG. 1, the stringers need not be resistant to a large bending moment since the bending moment that acts at the upper end of each height adjusting plate 4 is relatively small, as shown in FIG. 2. Therefore, the stringers 8 are attached to the height adjusting plates 4 by relatively simple means such as a fitting.
Thus, in the above-described conventional free access floor the bases 2 are secured to the floor surface 1 by means of an adhesive so that the free access floor is resistant to the moment that acts so as to collapse it by virtue of the bond strength of the adhesive. Therefore, the prior art involves the following problems. The bond strength of an adhesive is likely to change in accordance with bonding conditions, for example, the surface condition of the floor surface 1, and the bond strength lowers with age. Accordingly, a relatively large bond area is needed in order to obtain a predetermined level of aseismatic performance, resulting in an increase in the size of the bases 2. It is therefore difficult to improve the constructibility of the free access floor.
Since the foregoing free access floor needs the reinforcing rods 7, the number of steps required to construct the free access floor increases and, since the reinforcing rods 7 are disposed on the floor surface 1 at predetermined regular spacings, the space under the floor cannot be utilized to the fullest.
Further, although each pair of adjacent support legs 3 are connected together through a stringer 8, each stringer 8 and the corresponding height adjusting plates 4 are connected together simply by a fitting or the like and there is therefore a clearance between each stringer 8 and a height adjusting plate 4 fitted thereto. Accordingly, a load that is applied to one support leg 3 cannot be transmitted to another support leg 3 which is adjacent to it through a stringer 8 so that the load is distributed to a large number of support legs 3. For this reason, when a heavy object is placed on the free access floor, the load is concentrated on a relatively small number of support legs 3 and therefore these support legs 3 must be reinforced. In such a case, there are two different kinds of support legs 3 in the construction of such a free access floor, that is, those which need reinforcement and those which do not, and therefore the construction cannot be performed at high efficiency. In other words, the constructibility of the prior art floor has heretofore been unsatisfactory.